Small hollow glass spheres are desirable in many industries for use as fillers for various organic and inorganic matrices. With these spheres, special properties can be achieved such as low dielectric constant, high strength to weight ratio and low thermal conductivity.
In the electronics industry, small microspheres are desirable because conductor lines deposited on electronic substrates are small, and the spheres, in order to be effective, should be close to the dimensions of these conductor lines.
U.S. Pat. Nos. 2,978,339, 2,978,340 and 3,030,215 to Veatch et al. describe a method of forming hollow glass spheres from siliceous-based feed particles by using a bottom feed of particles into an ascending column of hot furnace gases. The preferred feed is an alkali metal silicate which contains an agent rendering the product glass spheres less sensitive to water. These agents include metal oxides such as oxides of zinc, aluminum, calcium, iron, boron, magnesium or lead. Also incorporated into the feed material is a "gasifying agent" to produce a low density product.
Such "gasifying agents" or "blowing agents" are substances that liberate gases at elevated temperatures. Examples of gasifying agents are carbonates, nitrates and various organics such as urea. The feed particles are heated in a furnace to obtain simultaneous glass formation at the time of triggering the blowing agent to cause expansion.
U.S. Pat. No. 3,365,315 to Beck et al. describes a process for the preparation of "glass bubbles" between 5 and 300 microns in diameter by "blowing" or expanding pre-formed glass particles of particular compositions. It is speculated that the "blowing" is caused by gases adsorbed within and on the pre-formed glass particles.
Hendricks, in U.S. Pat. No. 4,133,854, uses a droplet generator and a urea blowing agent to form hollow glass spheres about 20-1000.mu. in diameter.
Garnier et al. in U.S. Pat. No. 4,778,502 disclose a method where glass particles to be expanded are obtained by finely grinding a solid precursor glass. A fluidizing agent is added to the ground glass particles to prevent agglomeration of the particles before or during the expansion by heating.
It is difficult to achieve a high yield of the desired small sized hollow spheres using known solid feed or liquid feed methods of generating the particles to be expanded.
In the solid feed method, the size of the resultant spheres can be reduced by grinding to reduce the size of the feed particles. Grinding is generally disadvantageous because it generates large amounts of unusable fines and can introduce unwanted impurities. Additionally, such grinding represents an additional process step which adds to the product cost.
In the liquid feed process, the size of the resultant spheres might be reduced by adding large amounts of water to the feed. This technique would involve large amounts of excess liquid and/or be difficult to control.
Accordingly, there remains a need for a simple, economical process for producing a high yield of small hollow spheres.